In any area where radioactive materials are handled, it is imperative, both for the protection of personnel and to avoid contamination of the environment, to continuously monitor personnel, surfaces, equipment, and clothing to prevent the release of radioactive contamination. Alpha contaminants, such as plutonium and uranium, are particularly difficult to detect because plutonium primarily emits alpha radiation, and alpha radiation has very limited penetration in air. Alpha particles from typical contaminants travel no more than one inch in air. It is because of this characteristic that prior alpha detectors have been useful only when used in close proximity to the point of possible radioactive emission.
In the past, several instrument designs have been utilized to detect alpha radiation. Among these are GM tubes, ionization chambers, count rate detectors, and scintillation or gas flow proportional probes. While these instruments are capable of detecting alpha particles, they do so by directly detecting incident radiation, and must be within an inch of the source of the radiation. Also, these conventional alpha particle detectors can only scan an area approximately equal to the size of the detector.
Prior to the development of the long range alpha detector technology, alpha contamination, because of its short range in air, could not be detected if it originated in a space that was too small for insertion of a conventional monitor. Conventional detectors have normally been employed in personnel screening, when moved slowly in close proximity to a person's body. Workers in nuclear processing facilities must place their hands and feet on sensors when moving from room to room. All of this can slow operations, as it is not currently possible to adequately screen personnel within a reasonably short period of time.
It is also extremely difficult to monitor equipment and surfaces for alpha contamination, again due to the limited range of alpha particles in air. Because of the monitoring difficulty, equipment that has been used in a potentially contaminated area is often classified as potentially contaminated, and its further use is restricted to other controlled areas, or even discarded prior to the end of its useful life. If such equipment could be effectively monitored for contamination, the equipment could be released for use in uncontrolled areas. Previously, contamination inside assemblies has been impossible to detect without dismantling the assembly.
Additionally, within plutonium processing facilities and in many decontamination projects, potential alpha emitting contamination on the floor is a serious problem. Loose material is easily tracked from room to room, causing the contamination to spread. The conventional method of monitoring for this contamination was to employ floor monitors consisting of a portable alpha monitor with a very large detector head that would slowly scan over the floor.
These detectors suffered from numerous problems, among them being intrinsically poor sensitivity, degradation over time because of use by unskilled individuals, and workers moving the detector too rapidly over the surface. All of these problems are addressed by the present invention, which provides a sensitive, accurate and versatile alpha detector.
The present invention allows contamination detection from surfaces while at the same time canceling the effect of natural radioactive gases emanating from the soil surface as well as limiting the sensitivity of the measurement to external gamma ray sources.
As used herein, the terms "long range," or "long distance," when referring to the detection capabilities of the present invention, shall mean detection from a range or distance of more than one (1) inch from the source of alpha radiation.
The primary reason for an alpha particle's short flight path in air is its collision with air molecules. In almost all of these collisions, various of the molecular species in air are ionized. These ions, referred to herein as "air ions," have a sufficiently long lifetime that they may be transported by mass flow of the surrounding air, or by the direct attraction of an electric field, and detected at distances much greater than the penetration distances of the original alpha particles. That is, the air ions thus created have a longer life and area of influence than the alpha particles that created them. These are the ions that are detected by the present invention. The fact that the air ions have a longer range than the alpha particles relieves the necessity for having a detector moved in close proximity over a person or equipment in order to detect the presence of alpha radiation.
The present invention provides an alpha monitor with background cancellation. It is based on technology which is contained in several U.S. Patents which disclose various devices for the long range detection of alpha particles. The first is U.S. Pat. No. 5,184,019, issued Feb. 2, 1993, for a Long Range Alpha Particle Detector. The second is U.S. Pat. No. 5,194,737, issued Mar. 16, 1993, for Single and Double Grid Long Range Alpha Detectors. The third is U.S. Pat. No. 5,187,370, issued Feb. 16, 1993, for Alternating Current Long Range Alpha Particle Detectors. The fourth is U.S. Pat. No. 5,281,824, issued Jan. 25, 1994, for Radioactive Detection. The fifth is U.S. Pat. No. 5,311,025, issued May 10, 1994, for Fan-less Long Range Alpha Detector. Another recently filed application bears Ser. No. 08/833,020, filed Nov. 1, 1994, entitled "Event Counting Alpha Detector." As previously described, the principle underlying each of these patents and patent application is that alpha particles, although themselves of very short range in air, ionize various of the molecular species in air. The present invention modifies this apparatus to provide for reliable detection of alpha radiation from surfaces from which radioactive might be emitted. The invention employs background cancellation through the electrostatic detection of air ions created both by alpha contamination and by background radiation.
The fact that long range alpha detectors, as described in the above-referenced patents and application, can detect alpha radiation at a considerable distance from its point of emanation allows for monitoring of contamination in several areas which are extremely difficult or even impossible for current detectors. The current invention accomplishes this through the use of dual detection chambers, allowing emissions from the decay of radioactive gas and any daughters to be detected separately from emissions resulting from surface contamination.
It is therefore an object of the present invention to provide apparatus for the long range detection of alpha radiation emitted from a surface.
It is another object of the present invention to provide apparatus for the detection of alpha radiation emitted from a surface while at the same time canceling any background radiation originating from radioactive gas and any daughters or from strong external gamma radiation sources.
It is yet another object of the present invention to provide apparatus for the detection of alpha radiation emanating from a surface which can provide separate outputs indicative of the levels both of surface contamination and of radioactive emission.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.